1. Field of the Invention
The present invention relates to a steering device.
2. Description of the Related Art
There has hitherto been known an electric power steering (EPS) device in which a ball screw mechanism provides thrust in the axial direction to a rack shaft provided as a steered shaft to assist a driver in performing a steering operation. For example, Japanese Patent Application Publication No. 2012-144202 (JP 2012-144202 A) describes an EPS device in which a motor is attached in parallel with a rack shaft and a ball screw mechanism converts rotational torque of the motor into linear motion of the rack shaft to assist a driver in performing a steering operation. In this event, the rack shaft is urged to be rotated (in an accompanying manner) in response to the rotational torque of the motor. However, the rack shaft is supported by a rack-and-pinion mechanism, and prevented from rotating.
When rotation of the rack shaft is suppressed using only the rack-and-pinion mechanism, however, the surface pressure for contact between rack teeth and pinion teeth is made non-uniform by the rotational torque, and the teeth may be worn unevenly to cause a backlash or abnormal sound. In EPS devices for large vehicles which require larger rotational torque, in particular, the teeth may be worn unevenly to a further degree by the larger rotational torque. As one of measures for addressing such an issue, it is conceivable to support the rack shaft so that the rack shaft is non-rotatable by providing the back surface of the rack shaft, which is opposite to the rack teeth in the circumferential direction, with a Y-shape, and providing a support yoke of a rack guide with a shape for receiving the Y-shape. In this case, however, it is required to design and adjust the clearances of the support yoke of the rack guide in the axial direction and the radial direction with high precision, which takes a high cost and much trouble. Therefore, it is desired to support the rack shaft so that the rack shaft is non-rotatable with a simpler configuration even in the case where the rotational torque acts on the rack shaft.